1. Field of the Invention
The present invention relates to an optical waveguide device which comprises: an optical waveguide including a plurality of cores; and a light-receiving element including a plurality of photo diodes.
2. Description of the Related Art
Conventionally, an optical waveguide device which comprises: an optical waveguide including a plurality of cores; and a light-receiving element including a plurality of photo diodes is known (For example, U.S. Pat. No. 6,351,260 B1). In this kind of optical waveguide device, generally, one photo diode is arranged to correspond with one core. Light emitted from each core is received by each corresponding photo diode. Intensity of received light is converted into an electrical signal by each photo diode.
The aforementioned optical waveguide device is typically used for an optical touch panel. In an optical touch panel, light of a light source is blocked by a touch input, such as a finger or a pen and the like. The optical waveguide device detects the position where light intensity is reduced to specify coordinates of the finger or the pen and the like.
FIG. 4 (a) is a schematic plan view of a first example of a conventional optical waveguide device 20 and FIG. 4 (b) is a cross-sectional view of the optical waveguide device 20. The conventional optical waveguide device 20 comprises: an optical waveguide 21; and a light-receiving element 22. The optical waveguide 21 includes a plurality of cores 23. Each core 23 emits outgoing light 25 from each distal end 24 thereof. The light-receiving element 22 includes a plurality of aligned photo diodes 26. Each photo diode 26 receives the outgoing light 25 of the cores 23. Generally, respective pitches (central clearance) L6 between adjacent cores 23 are identical to respective pitches (central clearances) L7 between adjacent photo diodes 26. Consequently, the plurality of cores 23 correspond to the plurality of photo diodes 26 one-on one.
In general, the outgoing light 25 of the cores 23 travels while spreading out in a fan-like foam. Therefore, as shown in FIG. 4 (a), when distances L8 between distal ends 24 of the cores 23 and the light-receiving surfaces of the photo diodes 26 are long, the outgoing light 25 emitted from the cores 23 is not only incident on the photo diodes 26 facing the cores 23 but also on adjacent photo diodes 26.
FIG. 4 (a) illustrates this situation in detail. In FIG. 4 (a), the core 23 of No. 3 has no outgoing light. However, a portion of outgoing light 25 emitted from the core 23 of No. 2 and a portion of outgoing light 25 emitted from the core 23 of No. 4 are incident on the photo diode 26 of No. 3. The photo diode 26 of No. 3 has, however, weak incident light. Therefore, there are fears that the core 23 of No. 3 may be misjudged as having outgoing light regardless of no outgoing light of the core 23 of No. 3.
To avoid such a misjudgment, the threshold of each photo diode 26 needs to be increased so as not to detect incident light from the adjacent cores 23. When the threshold of the photo diode 26 is increased, light-receiving sensitivity of the optical waveguide device 20 is, however, deteriorated in an optical touch panel employing the optical waveguide device 20. This could cause defects that a touch input is not detected in the optical touch panel.
In a second example of a conventional optical waveguide device 30 as a countermeasure of the aforementioned problem, as shown in FIGS. 5(a) and 5(b), cores 31 and photo diodes 32 are caused to approach to shorten distances L9 between distal ends 33 of the cores 31 and light-receiving surfaces of the photo diodes 32. This makes it possible to prevent the light-receiving sensitivity of the optical waveguide device 30 from being deteriorated and to avoid defects of the touch input. This countermeasure, however, causes another problem.
As shown in FIG. 5 (b), in an optical waveguide 34, a core 31 is formed on an under-cladding layer 35 and is further embedded in an over-cladding layer 36. When a distance L9 between the distal end 33 of the core 31 and the light-receiving surface of the photo diode 32 is shortened, as shown in FIG. 5 (c), there is a possibility of the distal end 33 of the core 31 being exposed from the over-cladding layer 36 due to non-uniformity of the optical waveguide 34 at the time of production. In the case where the distal end 33 of the core 31 is exposed from the over-cladding layer 36, diffusive decay of outgoing light 37 of the core 31 will be remarkable. As a result, there are fears that optical transmittance from the core 31 to the photo diode 32 may become impossible. It is needed to improve the formation accuracy of the over-cladding layer 36 to prevent the distal end 33 of the core 31 from being exposed from the over-cladding layer 36. This causes a reduction in mass production capability of the optical waveguide 34.
Further, the conventional optical waveguide device 30 has a problem with optical axis alignment (core adjustments). FIG. 5 (a) shows a state in which each optical axis of respective cores 31 and respective photo diodes 32 has perfectly been aligned (the state in which core adjustments have been perfectly made). It is, however, difficult to perfectly make core adjustments to the cores 31 and the photo diodes 32, practically, as shown in FIG. 6 (a), there is a case where the center of the cores 31 and the center of the photo diodes 32 are misaligned. In the case of FIG. 6(a), an amount of deviation of the optical axes is L10.
As shown in FIG. 5 (a), when the center of the respective cores 31 and the center of the respective photo diodes 32 are not misaligned, the outgoing light of the core of No. 2 is not incident on the photo diode of No. 3. In the case of FIG. 6 (a), however, a portion of the outgoing light of the core of No. 2 is incident on the photo diode of No. 3 because the center of the core 31 and the center of the photo diode 32 are misaligned. Accordingly, there is weak incident light on the photo diode of No. 3. As a result, there is a possibility of being determined by mistake that there is outgoing light of the core of No. 3. This problem is not resolved, even when each distance L9 between the distal end 33 of the core 31 and the light-receiving surface of the photo diode 32 is shortened. To prevent the outgoing light 37 from being incident by mistake, the core adjustments in the core 31 and the photo diode 32 should be improved. This reduces mass production capability of the optical waveguide device 30.
In the conventional optical waveguide device 30, generally, the light-receiving area of the photo diodes 32 is greater than the emitting area of the cores 31 of the optical waveguide 34 (For instance, U.S. Pat. No. 6,351,260 B1, column 11, lines 56 to 62) so as to easily make optical axis adjustments (core adjustments). As the light-receiving area of the photo diode 32 is greater, respective pitches L7 between the adjacent photo diodes 32 become greater. In the conventional optical waveguide device 30, respective pitches L6 between adjacent cores 31 are identical to the respective pitches L7 between the adjacent photo diodes 32 and therefore, the respective pitches L6 between the adjacent cores 31 become greater. The problem of the mistaken determination caused by the amount L10 of deviation of optical axes between the cores 31 and the photo diodes 32 becomes easy to be solved by enlarging the respective pitches L6 between the adjacent cores 31 and the respective pitches L7 between the adjacent photo diodes 32. Such enlargement of the respective pitches L6 between the adjacent cores 31 and the respective pitches L7 between the adjacent photo diodes 32 makes it, however, difficult to improve the accuracy of the optical touch panel.